1. Field of the Invention
The present invention relates to a method of forming dielectric films, and in particular, to a method of forming dielectric films including a metal silicate suitable for use as high permittivity constant gate dielectric films in a semiconductor device.
2. Description of the Related Art
In recent years, the thinning of an equivalent oxide thickness (EOT) of gate dielectric films progresses accompanied by the miniaturization of the design rule of a LSI, and for example, according to the International Technology Road Map for Semiconductors (ITRS), it is considered necessary to achieve the EOT of 1 nm or less in 2008. However, when SiO2 or SiON used conventionally as the material of the gate dielectric film of the semiconductor devices are thinned to the level of the above described thickness, the insulation property of the film is rapidly lowered, and a leakage current becomes extremely large. This is because the dielectric films thickness becomes thin to the extent of several molecular layer portions, and direct tunnel current rapidly increases. On the other hand, in the mobile electronic equipments which have become rapidly widespread in recent years, to prolong the battery driving hours, it is requested to reduce the power consumption of the element as much as possible. Hence, the introduction of a high permittivity constant dielectric films (so-called High-K films) such as a metal oxide in place of SiO2 or SiON which has been used conventionally is under review. Since the High-K film has a high permittivity constant, even when its physical film thickness is made thick, it is possible to obtain the EOT equivalent to the SiO2 film, and moreover, it is possible to suppress the leak current to the lower level as the physical film thickness is thick.
As the material of the High-K films, it is said that hafnium nitride silicate (HfSiON) is currently most close to the practical application. As the film-forming method of the HfSiON film, a CVD method using an organic metal material, an atomic layer deposition method (ALD method), a sputtering method, and the like are under review. However, the sputtering method is advantageous in view of the fact that the mixture of carbon impurities is few.
Here, an example of the forming process of the HfSiON film using the sputtering method will be described by using FIGS. 6A to 6E. In FIGS. 6A to 6E, reference numeral 101 denotes a Si substrate, numeral 102 a SiO2 (silicon dioxide) film, numeral 105 a metal Hf film, numeral 107 HfSiO (Hf-silicate) film, and numeral 108 HfSiON film. First, on a cleaned Si substrate 101, a thin SiO2 film 102 is formed by thermal oxidation method, and upon thereof, a Hf atom is laminated by the sputtering method, and a metal Hf film 105 is formed. Next, radical oxidation by using the thermal oxidation or the oxygen plasma, and the like is performed. Thereby, the metal Hf film 105 is oxidized to introduce oxygen, and at the same time, a Hf atom in the metal Hf film 105 is dispersed into the SiO2 film 102, and moreover, a Si atom in the SiO2 film 102 is dispersed in the metal Hf film 105. Thus, the metal silicate film, that is, the HfSiO film 107 is formed. Thereby, the film thickness of the SiO2 film 102 of a low dielectric constant is reduced. After that, by performing radical nitriding using nitrogen plasma and the like, the HfSiO film 107 is nitrided to introduce nitrogen, thereby to form the HfSiON film 108. Because the relative permittivity constant of the HfSiON film 108 is high, and the film thickness of the SiO2 film 102 is reduced, a small dielectric film of the EOT including the HfSiON film 108 and the SiO2 film 102 is formed.
Now, the above described method involves a problem that a control of the diffusion of Hf is difficult, and that the formation of the dielectric films having a required film thickness with excellent productivity is difficult. That is, as disclosed in Japanese Patent Application Laid-Open No. 2002-314074 (US counterpart U.S. Pat. No. 6,734,069, US Patent Application Publication 2003/0092238), to expedite the diffusion, when the thermal oxidation is performed at high temperatures, a Si substrate of the ground is oxidized, and the thickness of the SiO2 film is sharply increased, and this makes it difficult to reduce the EOT. Further, even when a RTA (Rapid Thermal Annealing) and a flush lamp annealing are employed, the control of the diffusion is not easy. While the oxidation under high vacuum atmosphere makes it possible to disperse the film thickness while controlling the same, it takes a long time in the treatment, and causes a problem of the throughput being low. On the contrary, when the radical oxidation is performed at low temperature, since the diffusion is extremely sluggish, in the case where the oxidation treatment is performed for a short period, even after the oxidation, a thick SiO2 film remains, and this makes it difficult to form a small film of the EOT. Further, because the diffusion of the Hf is little, the finished film does not become a HfSiON film, but becomes a HfON film having a low heat resistance, so that crystallization occurs at an annealing treatment of the later process, thereby causing a problem of the leakage current being increased. Further, if the oxidation time is prolonged, though the diffusion progresses, the oxygen atom reaches up to the Si substrate earlier, and this causes a problem of the SiO2 film thickness becoming thick.
Meantime, Japanese Patent No. 3746968 (US counterpart U.S. Pat. No. 7,166,185, U.S. Pat. No. 7,374,635) discloses a method in which a silicon dioxide film is formed on the surface of a silicon substrate, and on the surface of this silicon dioxide film, a silicon nitride film is formed, and on the surface of this silicon nitride film, the high permittivity constant dielectric films are formed. Here, at the time of forming the silicon nitride film, the silicon dioxide film is exposed to nitride plasma. However, this nitride plasma exposure forms a silicon nitride film remained as the dielectric films by nitriding the surface of the silicon dioxide film, and this silicon nitride film isolates the silicon dioxide film and the high permittivity constant dielectric films.
As described above, in the film-formation of the High-K film, there is a problem that a control of the diffusion of Hf is difficult, and the formation of the dielectric films having a required film thickness with excellent productivity is difficult.